Photodetector and method having a conductive layer with etch susceptibility different from that of the semiconductor substrate

ABSTRACT

A photodetector includes semiconductor conductive layer, light-absorbing layer and wide bandgap layer, which are stacked in this order on a semi-insulating semiconductor substrate. The conductive layer has been formed on a surface region of the substrate. The photodetector further includes a doped region defined in part of the wide bandgap layer. The conductive layer has etch susceptibility different from that of the substrate.

Divisional of prior application Ser. No.: 09/862,504 filed May 23, 2001now U.S. Pat. No. 6,586,718.

BACKGROUND OF THE INVENTION

The present invention relates to a photodetector and more particularlyrelates to a high-speed photodetector with the capacitance at its padportion reduced by forming a mesa-shaped light-absorbing layer on partof a semi-insulating semiconductor substrate and an electrode pad ananother part thereof, respectively.

A photodetector for use in fiber optics communication, which exhibits aphotosensitivity to some incident radiation with a long wavelengthranging from 1.3 μm to 1.55 μm, may be implemented typically as a pinphotodiode made of InGaAs and InP compound semiconductors. A pinphotodiode of this type often has its response speed restricted by a CRtime constant, which is a product of the capacitance of the photodiodeand a load resistance. Accordingly, to increase the response speed of apin photodiode, the photodiode needs to have a reduced capacitance.

And to reduce the capacitance of a photodiode, not only the junctioncapacitance but also the capacitance associated with its electrode padshould be reduced as well. In high-speed photodetectors (or photodiodes)of today, in particular, the photodiode section thereof has a muchsmaller size. Accordingly, a ring electrode (typically with a diameterof about 35 μm) formed on the photodiode section is now smaller in sizethan an electrode pad (typically with a diameter of about 80 μm)extended from, and disposed near, the ring electrode. For that reason,the capacitance of the electrode pad has a considerable effect on theresponse speed of the photodiode. In a known structure speciallydesigned to reduce the pad capacitance, a thick insulating film ofpolyimide is interposed between the electrode pad and a semiconductorlayer. However, to further reduce and almost eliminate the padcapacitance, another known structure includes: a mesa-shapedlight-absorbing layer on part of a semi-insulating semiconductorsubstrate; and an electrode pad on another part thereof on which thelight-absorbing layer does not exist.

A photodetector with such a structure is disclosed, for example, inJapanese Laid-Open Publication No. 5-82829. FIG. 9 schematicallyillustrates the structure of the photodetector disclosed in thispublication.

The photodetector 500 shown in FIG. 9 includes photodiode mesa 505 andpad mesa 506 that are formed on a semi-insulating InP substrate 501.More specifically, the photodiode mesa 505 includes n⁺-InP, n⁻-InGaAsand n-InP layers 502, 503 and 504, which are stacked in this order onpart of the substrate 501. On the other hand, the pad mesa 506 alsoincludes the n⁺-InP, n⁻-InGaAs and n-InP layers 502, 503 and 504, whichare stacked in this order on another part of the substrate 501 where thephotodiode mesa 505 does not exist. A pad electrode 511 is formed on theupper surface of the pad mesa 506.

The photodiode mesa 505 further includes a p⁺-type doped region 507 thathas been formed by heavily doping part of the n-InP layer 504 with ap-type dopant so that the dopant reaches the InGaAs layer 503 as alight-absorbing layer. And an insulating film 510 of SiN has beendeposited over the substrate 501. A p-side electrode 508 is formed onthe insulating film 510 and is electrically connected to part of thedoped region 507. An n-side electrode 509 is also formed on theinsulating film 510 but is electrically connected to part of the n-InPlayer 504 where the doped region 507 does not exist. The p-sideelectrode 508 on the photodiode mesa 505 is connected to the padelectrode 511 on the pad mesa 506 by way of an interconnect 512 that hasbeen formed on the insulating film 510.

In the photodetector 500 shown in FIG. 9, part of the n⁺-InP layer 502,which existed between the photodiode and pad mesas 505 and 506originally, has been removed completely to electrically isolate thephotodiode and pad mesas 505 and 506 from each other. The n⁺-InP layer502 will be herein called a “semiconductor conductive layer”. However,the semiconductor conductive layer 502 and semiconductor substrate 501are both made of InP, and it is difficult to etch away that part aloneas intended. For that reason, in the known photodiode 500, thesemiconductor conductive layer 502 is etched rather deep and the surfaceof the substrate 501 is also etched away partially to remove that partof the semiconductor conductive layer 502 located between the photodiodeand pad mesas 505 and 506 completely. As a result, the photodiode andpad mesas 505 and 506 can be isolated electrically, but the respectiveheights of the mesas 505 and 506 as measured from the surface of thesubstrate 501 are higher than the originally designed ones.

The higher the photodiode and pad mesas 505 and 506, the harder it is toform the interconnect 512 and bridge these mesas 505 and 506 together asdesigned. This is because where the mesas 505 and 506 are so high, partof the interconnect 512 located around the corner between the photodiodeor pad mesa 505 and 506 and the substrate 501 easily peels off. That isto say, to form the interconnect 512 more reliably, the mesas 505 and506 should preferably have their heights reduced. In the knownphotodetector 500, however, the heights of the photodiode and pad mesas505 and 506 exceed the minimum required ones to completely isolate thesemesas 505 and 506 electrically from each other.

In addition, the photodetector 500 shown in FIG. 9 also has anon-negligibly large interconnect capacitance. In the photodetector 500,the pad electrode 511 and part of the interconnect 512 on the substrate501 create no parasitic capacitance. However, another part of theinterconnect 512 on the photodiode mesa 505 does create someinterconnect capacitance. Where the photodetector 500 should operate ata high speed with the area of the doped region 507 minimized, thisinterconnect capacitance is non-negligibly large compared to thejunction capacitance thereof. Particularly when the insulating film 510located between the interconnect 512 and the n-InP layer 504 (which willbe herein sometimes called a “window layer”) is made of a single SiNlayer, the interconnect capacitance increases noticeably. The reason isas follows. Firstly, the SiN layer should be thin enough because crackswould be formed easily otherwise. Also, an SiN film has a higherdielectric constant than that of any other insulating film (e.g., SiO₂film).

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aphotodetector that can be mass-produced easily.

Another object of this invention is to provide a high-performancephotodetector with an optical filtering function.

Still another object of the invention is to provide a high-speedphotodetector with a reduced interconnect capacitance.

An inventive photodetector includes semi-insulating semiconductorsubstrate, semiconductor conductive layer, light-absorbing layer, widebandgap layer and doped region. The conductive layer has been formed ona surface region of the substrate and has electrical conductivity. Thelight-absorbing layer has been formed on the conductive layer andabsorbs light that has been incident on the photodetector. The widebandgap layer has been formed on the light-absorbing layer and has abandgap wider than that of the light-absorbing to layer. And the dopedregion has been defined in the wide bandgap layer by doping part of thewide bandgap layer with a dopant that reaches the light-absorbing layer.In this photodetector, the conductive layer has etch susceptibilitydifferent from that of the substrate.

In the photodetector according to a first aspect of the presentinvention, the conductive layer has etch susceptibility different fromthat of the substrate. Accordingly, by using an etchant (e.g., anetchant containing hydrochloric acid) that etches the conductive layerselectively with respect to the substrate, the conductive layer can beetched just as intended. That is to say, the etch process may be stoppedas soon as the surface of the substrate is exposed. Thus, the etchprocess is controllable much more easily and there is no need to removethe uppermost part of the substrate. Accordingly, the mesas do notincrease their heights too much. As a result, the interconnect can beformed easily and the photodetector of this type is mass-producible.Also, where a semiconductor multilayer structure (i.e., photodiodemesa), including the semiconductor conductive, light-absorbing and widebandgap layers, has stepped side faces, a photoresist pattern,determining the shape of the interconnect, can be defined bettercompared to a mesa with no stepped side faces. Thus, the interconnectcan be formed even more satisfactorily.

In one embodiment of the present invention, the substrate, conductivelayer, light-absorbing layer and wide bandgap layer may be made of InP,InGaAsP, InGaAs and InP, respectively.

In another embodiment, InGaAsP as a material for the conductive layermay have an absorption edge longer than 0.93 μm and shorter than 1.55μm.

In still another embodiment, the conductive layer may be an n-typesemiconductor layer, the dopant may be a p-type dopant and thelight-absorbing layer may function as an intrinsic layer of a pinphotodiode. And the photodetector may further include: an n-sideelectrode, which makes an electrical contact with the conductive layer;and a p-side electrode, which makes an electrical contact with the dopedregion.

In yet another embodiment, a semiconductor multilayer structure,including the semiconductor conductive, light-absorbing and wide bandgaplayers, may have been formed on said surface region of the substrate. Asecond semiconductor conductive layer may have been formed on anothersurface region of the substrate and may be electrically isolated fromthe conductive layer included in the multilayer structure. A pad for useto electrically connect the photodetector to an external unit may havebeen formed on the second conductive layer. And the pad may beelectrically connected to the doped region that has been defined in saidpart of the wide bandgap layer in the multilayer structure.

In this particular embodiment, a ring electrode with an opening at thecenter thereof has preferably been formed on the doped region. And thering electrode is preferably connected to the pad by way of aninterconnect that has been formed on an insulating film. The insulatingfilm preferably covers the surface of the multilayer structure.

Alternatively or additionally, the semiconductor conductive,light-absorbing and wide bandgap layers, making up the multilayerstructure, have preferably been stacked one upon the other to make alevel difference exist between each of these layers and an adjacent oneof the layers.

Another inventive photodetector includes semi-insulating semiconductorsubstrate, semiconductor conductive layer, light-absorbing layer,carrier barrier layer, wide bandgap layer and doped region. Theconductive layer has been formed on a surface region of the substrateand has electrical conductivity. The light-absorbing layer absorbs lightthat has been incident on the photodetector. The carrier barrier layerhas been formed between the conductive and light-absorbing layers toprevent carriers, created in the conductive layer, from diffusing andentering the light-absorbing layer. The wide bandgap layer has beenformed on the light-absorbing layer and has a handgap wider than that ofthe light-absorbing layer. And the doped region has been defined in thewide bandgap layer by doping part of the wide bandgap layer with adopant that reaches the light-absorbing layer. In this photodetector,the conductive layer is made of InGaAsP and transmits part of theincident light with a particular wavelength.

In the photodetector according to a second aspect of the presentinvention, the barrier layer is formed between the conductive andlight-absorbing layers. Accordingly, this photodetector can receive andsense light that has been incident through the backside thereof. Inaddition, the conductive layer, made of InGaAsP, can selectivelytransmit light with a particular wavelength out of the incoming light.Accordingly, where the incoming light has two wavelengths of 1.3 μm and1.55 μm, the photodetector may sense part of the incoming light with thelatter wavelength of 1.55 μm. In this manner, a high-performancephotodetector with an optical filtering function is realized.

In one embodiment of the present invention, InGaAsP as a material forthe conductive layer may have an absorption edge longer than 1.3 μm andshorter than 1.55 μm.

More specifically, the absorption edge is preferably longer than 1.35 μmand shorter than 1.5 μm.

In an alternative embodiment, InGaAsP as a material for the conductivelayer may also have an absorption edge longer than 0.93 μm and shorterthan 1.3 μm.

More particularly, the absorption edge is preferably longer than 0.93 μmand shorter than 1.25 μm.

In another embodiment of the present invention, the substrate and theconductive, barrier, light-absorbing and wide bandgap layers may be madeof InP, InGaAsP, InP, InGaAs and InP, respectively.

In still another embodiment, the photodetector may sense light that hasbeen incident on the photodetector through a backside of the substrate.

In yet another embodiment, a semiconductor multilayer structure,including the semiconductor conductive, carrier barrier, light-absorbingand wide bandgap layers, may have been formed on said surface region ofthe substrate. A second semiconductor conductive layer may have beenformed on another surface region of the substrate and may beelectrically isolated from the conductive layer included in themultilayer structure. A pad for use to electrically connect thephotodetector to an external unit may have been formed on the secondconductive layer. And the pad may be electrically connected to the dopedregion that has been defined in said part of the wide bandgap layer inthe multilayer structure.

Still another inventive photodetector includes semi-insulatingsemiconductor substrate, semiconductor conductive layer, light-absorbinglayer, wide bandgap layer, doped region and electrode. The conductivelayer has been formed on a surface region of the substrate and haselectrical conductivity. The light-absorbing layer has been formed onthe conductive layer and absorbs light that has been incident on thephotodetector. The wide bandgap layer has been formed on thelight-absorbing layer and has a bandgap wider than that of thelight-absorbing layer. The doped region has been defined in the widebandgap layer by doping part of the wide bandgap layer with a dopantthat reaches the light-absorbing layer. And the electrode has beenformed on the doped region. In this photodetector, a semiconductormultilayer structure, including the semiconductor conductive,light-absorbing and wide bandgap layers, has been formed on said surfaceregion of the substrate. A second semiconductor conductive layer hasbeen formed on another surface region of the substrate and iselectrically isolated from the conductive layer included in themultilayer structure. A pad for use to electrically connect thephotodetector to an external unit has been formed on the secondconductive layer. The multilayer structure is covered with an insulatingfilm. An interconnect has been formed on the insulating film toelectrical connect the electrode and the pad together. And theinsulating film is a stack of an SiN layer and an SiO₂ layer that hasbeen deposited on the SiN layer.

In the photodetector according to a third aspect of the presentinvention, the insulating film, formed on the surface of the multilayerstructure (i.e., photodiode mesa), is a stack of SiN and SiO₂ layers.Accordingly, the interconnect capacitance, formed between theinterconnect on the insulating film and the multilayer structure, can bereduced compared to a structure in which the insulating film is made ofa single SiN layer. As a result, a high-speed photodetector with areduced interconnect capacitance is realized.

In one embodiment of the present invention, the SiN layer may have athickness of 20 nm through 100 nm, and the SiO₂ layer may have athickness of 400 nm or more.

In another embodiment of the present invention, the photodetector mayfurther include a carrier barrier layer between the conductive andlight-absorbing layers. The barrier layer prevents carriers, created inthe conductive layer, from diffusing and entering the light-absorbinglayer.

An inventive method for fabricating a photodetector includes the step ofa) stacking semiconductor conductive, light-absorbing and wide bandgaplayers in this order on a semi-insulating semiconductor substrate by acrystal growth process. The conductive layer has etch susceptibilitydifferent from that of the substrate. The light-absorbing layer absorbsincoming light. And the wide bandgap layer has a bandgap wider than thatof the light-absorbing layer. The method further includes the steps ofb) defining a doped region in part of the wide bandgap layer by dopingsaid part with a dopant that reaches the light-absorbing layer; c)etching and patterning the wide bandgap and light-absorbing layers intorespectively predetermined shapes; d) defining an etch mask on theconductive layer so that the wide bandgap and light-absorbing layers inthe predetermined shapes are covered with the mask; and e) selectivelyremoving part of the conductive layer using an etchant that etches saidpart of the conductive layer away with respect to the substrate.

In one embodiment of the present invention, the step c) may include thesteps of: i) defining a first etch mask on the wide bandgap layer sothat the doped region is covered with the first mask after the step b)has been performed; ii) selectively etching part of the wide bandgaplayer away with respect to the light-absorbing layer; iii) defining asecond etch mask on the light-absorbing layer so that the wide bandgaplayer is covered with the second mask; and iv) selectively etching partof the light-absorbing layer away with respect to the conductive layer.

Specifically, the substrate and the conductive, light-absorbing and widebandgap layers may be made of InP, InGaAsP, InGaAs and InP,respectively. And the etchant may contain hydrochloric acid.

In an alternative embodiment, the substrate and the conductive,light-absorbing and wide bandgap layers may be made of InP, InGaAsP,InGaAs and InP, respectively. And the steps ii) and iv) may be performedusing an etchant containing sulfuric acid.

Another inventive method for fabricating a photodetector includes thestep of a) stacking semiconductor conductive, carrier barrier,light-absorbing and wide bandgap layers in this order on asemi-insulating semiconductor substrate by a crystal growth process. Theconductive layer has electrical conductivity. The carrier barrier layerprevents carriers, created in the conductive layer, from diffusing andentering upper layers thereof. The light-absorbing layer absorbsincoming light. And the wide bandgap layer has a bandgap wider than thatof the light-absorbing layer. The method further includes the steps of:b) defining a doped region in part of the wide bandgap layer by dopingsaid part with a dopant that reaches the light-absorbing layer; c)defining a first etch mask on the wide bandgap layer so that the dopedregion is covered with the first mask; d) selectively etching part ofthe wide bandgap layer away with respect to the light-absorbing layerusing a first etchant; e) defining a second etch mask on thelight-absorbing layer so that the wide bandgap layer is covered with thesecond mask; f) selectively etching part of the light-absorbing layeraway with respect to the barrier layer using a second etchant; g)selectively etching part of the barrier layer away with respect to theconductive layer using a third etchant; h) defining a third etch mask onthe conductive layer so that the wide bandgap, light-absorbing andcarrier barrier layers are covered with the third mask; and i)selectively etching part of the conductive layer away with respect tothe substrate using a fourth etchant.

In one embodiment of the present invention, the substrate and theconductive, barrier, light-absorbing and wide bandgap layers may be madeof InP, InGaAsP, InP, InGaAs and InP, respectively. The first and thirdetchants may contain hydrochloric acid, while the second and fourthetchants may contain sulfuric acid.

Still another inventive method for fabricating a photo-detector includesthe step of a) stacking semiconductor conductive, light-absorbing andwide bandgap layers in this order on a semi-insulating semiconductorsubstrate by a crystal growth process. The conductive layer has etchsusceptibility different from that of the substrate. The light-absorbinglayer absorbs incoming light. And the wide bandgap layer has a bandgapwider than that of the light-absorbing layer. The method furtherincludes the steps of b) defining a doped region in part of the widebandgap layer by doping said part with a dopant that reaches thelight-absorbing layer; c) etching and patterning the wide bandgap andlight-absorbing layers into respectively predetermined shapes; d)selectively etching part of the conductive layer away, thereby defininga semiconductor multilayer structure, which includes the wide bandgapand light-absorbing layers in the predetermined shapes and theconductive layer, and leaving a second part of the conductive layer sothat the second part serves as a second semiconductor conductive layerspaced apart from the conductive layer included in the multilayerstructure; e) depositing SiN and SiO₂ layers in this order over thesurface of the multilayer structure, exposed parts of the substrate andthe second conductive layer, thereby forming an insulating filmincluding the SiN and SiO₂ layers; f) removing part of the insulatingfilm, which is located over the doped region in the wide bandgap layerincluded in the multilayer structure, thereby forming an opening overthe doped region; g) forming an electrode on part of the doped regioninside the opening; h) forming a pad for use to electrically connect thephotodetector to an external unit on either part of the insulating filmthat has been formed on the exposed part of the substrate or anotherpart of the insulating film that has been formed over the secondconductive layer; and i) forming an interconnect on the insulating filmto electrically connect the electrode and the pad together.

In one embodiment of the present invention, the steps g), h) and i) maybe performed as a single process step.

In this particular embodiment, the single process step preferablyincludes: depositing a spacer film of SiN on the insulating film;defining a negative photoresist pattern on the spacer film to form theelectrode, the pad and the interconnect; etching parts of the spacerfilm away using the photoresist pattern as a mask; depositing a metal onexposed parts of the insulating film and on the photoresist pattern,thereby forming a metal thin film thereon; and lifting the photoresistpattern off along with excessive parts of the metal on the photoresistpattern, thereby forming the electrode, the pad and the interconnect.

In an inventive photodetector, a semiconductor conductive layer has etchsusceptibility different from that of a semiconductor substrate.Accordingly, a semiconductor multilayer structure, including theconductive layer, does not increase its height too much. As a result, aninterconnect can be formed easily and the photodetector of this type ismass-producible.

In another inventive photodetector, a carrier barrier layer is furtherformed between semiconductor conductive and light-absorbing layers.Accordingly, this photodetector can sense light that has been incidentthrough the backside thereof. In addition, the conductive layer canselectively transmit incoming light with a particular wavelength. As aresult, a high-performance photodetector with an optical filteringfunction (i.e., wavelength selectivity) is realized.

In a third inventive photodetector, an insulating film, deposited on thesurface of a semiconductor multilayer structure, is a stack of SiN andSiO₂ layers. As a result, a high-speed photodetector with a reducedinterconnect capacitance is realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating aphotodetector according to a first embodiment of the present invention.

FIGS. 2A through 2D are cross-sectional views illustrating respectiveprocess steps for fabricating the photodetector of the first embodiment.

FIG. 3 is a graph illustrating a relationship between the wavelength andthe intensity of light absorbed into InGaAsP compounds.

FIG. 4 is a cross-sectional view schematically illustrating aphotodetector according to a second embodiment of the present invention.

FIG. 5 is a band diagram illustrating the bandgaps of respective layersincluded in the photodetector of the second embodiment.

FIGS. 6A through 6D are cross-sectional views illustrating respectiveprocess steps for fabricating the photodetector of the secondembodiment.

FIG. 7 is a cross-sectional view schematically illustrating aphotodetector according to a third embodiment of the present invention.

FIGS. 8A through 8D are cross-sectional views illustrating respectiveprocess steps for fabricating the photodetector of the third embodiment.

FIG. 9 is a cross-sectional view schematically illustrating a knownphotodetector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings, in which eachmember having substantially the same functions will be identified by thesame reference numeral for the sake of simplicity of description. Itshould be noted that the following embodiments of the present inventionare nothing but illustrative ones, and the present invention is in noway limited to these specific embodiments.

Embodiment 1

A first embodiment of the present invention will be described withreference to FIGS. 1 through 3. FIG. 1 schematically illustrates across-sectional structure for a photodetector 100 according to the firstembodiment.

The photodetector 100 of the first embodiment is a pin photodiodeincluding semiconductor layers of InGaAs and InP compounds. As shown inFIG. 1, the photodetector 100 includes semiconductor conductive layer102, light-absorbing layer 103 and wide bandgap layer 104, which havebeen stacked in this order on a semi-insulating semiconductor substrate101.

The conductive layer 102 with etch susceptibility different from that ofthe substrate 101 has been formed on a surface region of the substrate101. In the illustrated embodiment, the substrate 101 is asemi-insulating InP substrate. The conductive layer 102 is made ofInGaAsP, which is different from the composition of the InP substrate101. And the conductive layer 102 serves as an n-layer for the pinphotodiode.

The light-absorbing layer 103 on the conductive layer 102 absorbs lightthat has been incident on the photodetector 100. In the illustratedembodiment, the light-absorbing layer 103 is made of n⁻-InGaAs andserves as an i-layer for the pin photodiode. The light-absorbing layer103 exists on part of the conductive layer 102. On another part of theconductive layer 102 where the light-absorbing layer 103 does not exist,an n-side electrode 107 has been formed.

The wide bandgap layer 104 on the light-absorbing layer 103 has abandgap wider than that of the light-absorbing layer 103. And in part(e.g., center) of the wide bandgap layer 104, a p⁺-type doped region 105has been defined by doping that part with a dopant (e.g., Zn) thatreaches the light-absorbing layer 103. In the illustrated embodiment,the wide bandgap layer 104 is an n⁻-InP layer.

The wide bandgap layer 104 has a bandgap wider than that of thelight-absorbing layer 103. Accordingly, the wide bandgap layer 104 cantransmit light that has such a wavelength as getting the light absorbedinto the light-absorbing layer 103. That is to say, the wide bandgaplayer 104 can serve as a window layer for the pin photodiode. If thep-type dopant (e.g., Zn in this embodiment) is further diffused to makethe wide bandgap layer 104 a main p-type layer, then almost all of thelight-absorbing layer 103 can be an i-layer, thus increasing thephotosensitivity of the photodetector 100. The function of the widebandgap layer 104 as a window layer is particularly important where thephotodetector 100 is supposed to sense incoming light through theprincipal surface thereof.

The wide bandgap layer 104 not just functions as a window layer but alsoreduces a reverse leakage current flowing through the pn junction. Thelatter function of the wide bandgap layer 104 counts not only where thephotodetector 100 senses light incoming through the principal surfacethereof but also where the photodetector 100 senses light incidentthrough the backside thereof. It is known that if the pn junction isexposed on the surface of the InGaAs layer (i.e., the light-absorbinglayer 103) in an InGaAs/InP pin photodiode, then the leakage currentincreases. However, where the wide bandgap layer 104 of InP exists onthe InGaAs layer 103 as in the illustrated embodiment, the pn junctionwill be exposed on the surface of the wide bandgap layer 104, not on thesurface of the InGaAs layer 103. Accordingly, the leakage current can bereduced compared to a photodetector including no wide bandgap layer 104.

On the p⁺-type doped region 105 in the wide bandgap layer 104, a p-sideelectrode 106 has been formed. In the illustrated embodiment, the p-sideelectrode 106 has a multilayer structure consisting of Ti, Pt and Aufilms and is formed as a ring electrode disposed along the outerperiphery of the cylindrical doped region 105. A ring electrode with anopening at its center is used as the p-side electrode 106 to prevent theelectrode 106 from blocking the incoming light and thereby sense theincoming light as efficiently as possible.

The p-side electrode 106 is electrically connected to a pad electrode110 for use to electrically connect this photodetector 100 to anexternal unit (not shown). In the illustrated embodiment, the conductivelayer 102, light-absorbing layer 103 and wide bandgap layer 104 with thedoped region 105 are all included in a semiconductor multilayerstructure (i.e., photodiode mesa) 140. That is to say, the p-sideelectrode 106 is located on the top of the photodiode mesa 140, whilethe pad 110, electrically connected to the p-side electrode 106, islocated on another surface region of the substrate 101 where thephotodiode mesa 140 does not exist.

In the embodiment illustrated in FIG. 1, a second semiconductorconductive layer 109 has been formed on the substrate 101. The secondconductive layer 109 is spaced apart from the conductive layer 102included in the photodiode mesa 140 (which will be called a “firstconductive layer” for convenience sake) but is made of the same materialas the first conductive layer 102. And the pad 110 is disposed over thesecond conductive layer 109 with an insulating film 108 interposedtherebetween. By spacing the pad 110 apart from the photodiode mesa 140in this manner, a parasitic capacitance associated with the pad 110 canbe reduced.

In the illustrated embodiment, the second conductive layer 109 is usedas a pad mesa 160 for mounting the pad 110 thereon. Then, the n-sideelectrode 107 on the first conductive layer 102 can be substantiallyleveled with the pad 110 that is electrically connected to the p-sideelectrode 106. Where the n-side electrode 107 and pad 110 are positionedat substantially the same level, the photodetector 100 can be mountedonto a motherboard with connection members (e.g., bumps) of a sizeplaced on the electrode 107 and pad 110. Thus, the photodetector 100 canbe mounted much more easily.

The photodiode mesa 140, pad mesa 160 and substrate 101 are entirelycovered with the insulating film 108 of SiN, for example, except theregions where the n- and p-side electrodes 107 and 106 are located. Andon this insulating film 108, an interconnect 111 for electricallyconnecting the p-side electrode 106 to the pad 110 has been formed. Inthe illustrated embodiment, the p-side electrode 106, pad 110 andinterconnect 111 have been formed out of a single multilayer structureconsisting of Ti, Pt and Au films.

Also, to define a negative photoresist pattern for the interconnect 111in a good shape during the fabrication process, the side faces of thephotodiode mesa 140 are stepped. That is to say, a level differenceexists between each adjacent pair of the layers 102, 103 and 104included in the photodiode mesa 140. In other words, the upper surfaceof each underlying layer 102 or 103 included in the photodiode mesa 140is greater in area than the lower surface of the layer 103 or 104located on the underlying layer 102 or 103.

The photodetector 100 of the first embodiment may be formed in thefollowing specific shape. However, the present invention is not limitedto these specifics but any other appropriate combination may be usedinstead.

The InP substrate 101 may have a thickness between about 150 μm andabout 200 μm. The photodiode mesa 140 may have a height of about 5 μm toabout 10 μm. The first conductive layer 102 may have a thickness ofabout 1.5 μm to about 3 μm and may be formed as a rectangular mesa withan area ranging from (40×80) μm² through (100×200) μm². Like the firstconductive layer 102, the second conductive layer 109 may also have athickness of about 1.5 μm to about 3 μm. The mesa of the secondconductive layer 109 may be either in a circular shape with a diameterof about 50 μm to about 100 μm or in a rectangular (or square) shape,each side of which is approximately 50 to 100 μm long.

The light-absorbing layer 103 may have a thickness of about 1.5 μm toabout 3 μm and the mesa thereof may be in a circular shape with adiameter of about 35 μm to about 70 μm. The wide bandgap layer 104 mayhave a thickness of about 1 μm to about 2 μm, and the mesa thereof maybe in a circular shape with a diameter of about 30 μm to about 60 μm.The doped region 105 in the wide bandgap layer 104 may also be in acircular shape with a diameter of about 25 μm to about 50 μm. It shouldbe noted that these members may be formed in the exemplified shapes whenviewed from over the photodetector 100 (i.e., in the direction parallelto a normal to the substrate surface).

The carrier densities of these layers 102, 103 and 104 may be asfollows. Specifically, the first conductive layer 102 of n⁺-type mayhave a relatively high carrier density, while the light-absorbing layer103 of n⁻- or i-type may have a relatively low carrier density. In theillustrated embodiment, the wide bandgap layer 104 is an n⁻-type layerwith a relatively low carrier density. However, the carrier density ofthe wide bandgap layer 104 is not particularly limited. In thephotodetector 100 of this embodiment, the conductive layer 102 has etchsusceptibility different from that of the semi-insulating InP substrate101. For that reason, the conductive layer 102 can be etchedselectively. That is to say, the etch process may be stopped as soon asthe surface of the InP substrate 101 is exposed. Thus, the etch processis controllable much more easily and there is no need to remove theuppermost part of the InP substrate 101. Accordingly, the photodiodemesa 140 does not increase its height too much. As a result, theinterconnect 111 can be formed easily and the photodetector 100 ismass-producible. Also, since the photodiode mesa 140 has stepped sidefaces, a negative photoresist pattern, determining the shape of theinterconnect 111, can be defined better compared to a mesa with nostepped side faces. Thus, the photodetector 100 excels in massproductivity in this respect also.

In the foregoing illustrative embodiment, the present invention isimplemented as the photodetector 100 including the stepped photodiodemesa 140. However, the present invention is not limited to such aspecific structure, but the photodiode mesa 140 may be formed in anyother appropriate shape. Also, in the foregoing embodiment, the p- andn-side electrodes 106 and 107 are formed on the doped region 105 andconductive layer 102, respectively. Nevertheless, to make the pinphotodiode operable, the p- and n-side electrodes 106 and 107 only needto be electrically connectible to the light-absorbing layer 103.Accordingly, the locations of these electrodes 106 and 107 are notlimited to those illustrated in FIG. 1, either. Thus, it should beeasily understandable to those skilled in the art that the photodetector100 is further modifiable in various ways.

Hereinafter, it will be described with reference to FIGS. 2A through 2Dhow to fabricate the photodetector 100 of the first embodiment. FIGS. 2Athrough 2D are cross-sectional views illustrating respective processsteps for fabricating the photodetector 100.

First, as shown in FIG. 2A, semiconductor conductive layer 102 ofn-InGaAsP, light-absorbing layer 103 of n⁻-InGaAs, and window layer(i.e., wide bandgap layer) 104 of n⁻-InP are stacked in this order on asemi-insulating semiconductor substrate 101 of InP by a crystal growthprocess. Next, part of the wide bandgap layer 104 is doped with a dopant(e.g., Zn) that reaches the light-absorbing layer 103, thereby defininga doped region 105.

Specifically, InGaAsP as a material for the conductive layer 102 may berepresented by In_(1−x)Ga_(x)As_(y)P_(1−y), where y=2.12x. That is tosay, the composition In_(1−x)Ga_(x)As_(y)P_(1−y), has a degree offreedom of one. Accordingly, when the mole fraction x or y is defined, acomposition InGaAsP with a predetermined absorption edge is determinedautomatically. In other words, the composition InGaAsP can berepresented by the absorption edge thereof.

FIG. 3 is a graph illustrating a relationship between the wavelength andthe intensity of light absorbed into the InGaAsP compounds. As usedherein, the absorption edge of In_(1−x)Ga_(x)As_(y)P_(1−y) (where 0≦x≦1and 0≦y≦1) means a maximum wavelength, at and below which light can beabsorbed thereto. As can be seen from FIG. 3, InP, not including Ga orAs (i.e., where x=0 and y=1 for In_(1−x)Ga_(x)As_(y)P_(1−y)), has anabsorption edge of 0.93 μm, which is the shortest of all InGaAsPcompounds. On the other hand, In_(1−x)Ga_(x)As, not including P (i.e.,where y=1 for In_(1−x)Ga_(x)As_(y)P_(1−y)) has an absorption edgeslightly longer than 1.6 μm, which is the longest of all InGaAsPcompounds. In this manner, an absorption edge for an InGaAsP compoundmay be arbitrarily selected from the range between these upper and lowerlimits by setting the mole fraction x and y appropriately.

The following Table 1 shows relationships between the absorption edgesλg of In_(1−x)Ga_(x)As_(y)P_(1−y), and the mole fractions x and y alongwith the bandgaps Eg:

TABLE 1 Absorption Edge λg Bandgap Mole Mole (μm) Eg (eV) Fraction xFraction y Note 0.92 1.35 0.00 0.00 InP 1.00 1.24 0.07 0.16 1.25 0.990.26 0.55 1.30 0.95 0.29 0.61 In_(0.71)Ga_(0.29)As_(0.61)P_(0.39) 1.350.92 0.32 0.68 1.50 0.83 0.40 0.85 1.55 0.80 0.42 0.90 1.65 0.75 0.471.00 In_(0.53)Ga_(0.47)As

As can be seen, by setting the mole fractions x and y arbitrarily, anInGaAsP layer 102 with an absorption edge of 1.0 μm, 1.30 μm, 1.5 μm,etc., can be obtained. To make the etch susceptibility of the n-InGaAsPlayer 102 different from that of the InP substrate 101, an n-InGaAsPlayer 102 with an absorption edge λg longer than 0.93 μm and shorterthan 1.55 μm may be used. To widen the difference in etchsusceptibility, an n-InGaAsP layer 102 with an absorption edge λg of 1.0μm or more should preferably be used.

In the foregoing embodiment, the wide bandgap layer 104 is made of ann-InP layer with λg of 0.92 μm. Accordingly, light with a wavelength of1.3 or 1.55 μm, which is often used in fiber optics communications, isnot absorbed into, but transmitted through, the wide bandgap layer 104.Thus, where the photodetector 100 senses light incoming through theprincipal surface thereof, the wide bandgap layer 104 can function as awindow layer.

Next, as shown in FIG. 2B, a first etch mask 201, covering the dopedregion 105, is defined on the wide bandgap layer 104 and then the widebandgap layer 104 is selectively etched while being masked with thefirst mask 201. The first mask 201 may be made of SiN, for example. Inthe illustrated embodiment, when the wide bandgap layer 104 should beetched, a first etchant, realizing an etch selectivity of 10 or more forthe wide bandgap layer 104 of InP against the light-absorbing layer 103of InGaAs, is used. Examples of such etchants include a mixture ofhydrochloric acid and phosphoric acid with a volume ratio of 1 to 4.Thereafter, the first mask 201 is removed.

Then, as shown in FIG. 2C, a second etch mask 202, covering the widebandgap layer 104 including the doped region 105, is defined on aselected area of the light-absorbing layer 103. Subsequently, thelight-absorbing layer 103 is selectively etched while being masked withthe second mask 202, which may also be made of SiN.

In the illustrated embodiment, when the light-absorbing layer 103 ofInGaAs should be etched, a second etchant, realizing an etch selectivityof 10 or more for the light-absorbing layer 103 of InGaAs against theconductive layer 102 of InGaAsP, is used. Examples of such etchantsinclude a mixture of sulfuric acid, hydrogen peroxide water and waterwith a volume ratio of 1 to 1 to 5. Thereafter, the second mask 202 isremoved.

Subsequently, as shown in FIG. 2D, a third etch mask 203 is defined onthe conductive layer 102 to cover not only the wide bandgap andlight-absorbing layers 104 and 103 but also part of the conductive layer102 where the mesa-etched light-absorbing layer 103 does not exist. Andthen the conductive layer 102 is masked with this third mask 203 andpartially etched away. The third mask 203 may also be made of SiN.

In the illustrated embodiment, when the conductive layer 102 of InGaAsPshould be etched, a third etchant, realizing an etch selectivity of 10or more for the conductive layer 102 of InGaAsP against the substrate101 of InP, is used. Examples of such etchants include a mixture ofsulfuric acid, hydrogen peroxide water and water with a volume ratio of5 to 1 to 1.

By performing these etch process steps, a photodiode mesa 140 withstepped side faces and a pad mesa 160 made of the second conductivelayer 109 are formed. Thereafter, the third mask 203 is removed. Andthen an insulating film 108 of SiN, for example, is deposited over thesubstrate except the area where the doped region 105 should be exposedand the area where an n-side electrode 107 should be formed. Thereafter,a negative photoresist pattern for p- and n-side electrodes 106 and 107,pad 110 and interconnect 111 is defined. Next, a stack of Ti, Pt and Aufilms is deposited over the substrate and then the photoresist patternis lifted off, thereby forming the p- and n-side electrodes 106 and 107,pad 110 and interconnect 111. In this manner, the photodetector 100shown in FIG. 1 is completed.

In the fabrication process of the first embodiment, the conductive layer102 of InGaAsP with etch susceptibility different from that of the InPsubstrate 101 is formed on the substrate 101, and can be etchedselectively with respect to the substrate 101. Thus, the etch process iscontrollable much more easily and there is no need to remove theuppermost part of the substrate 101 for the purpose of isolating theconductive layer 102 electrically. Accordingly, the photodiode mesa 140can have its height minimized as measured from the surface of thesubstrate 101.

In the fabrication process of this embodiment, the respectivesemiconductor layers are etched selectively using a number of etch masksin mutually different shapes. By using these multiple etch masks, thesemiconductor layers 104, 103 and 102 on the substrate 101 can be etchedsequentially so as to have upwardly decreasing planar areas. That is tosay, the respective semiconductor layers can be shaped in such a mannerthat the photodiode mesa 140 has stepped side faces.

If the negative photoresist pattern is deposited over a non-steppedphotodiode mesa 140, then the photoresist pattern will be relativelythick on the lower part of the mesa 140 but relatively thin on the upperpart thereof. When such a thin photoresist pattern is defined on theupper part of the mesa 140, then the excessive portions of the stackedmetal films, existing on the thin photoresist pattern, cannot be liftedoff as intended. As a result, the upper part of the mesa 140 will beentirely covered with those excessive metals. Where the photodiode mesa140 has stepped side faces as in this embodiment, that thin photoresistpattern, causing the pattern failure, is much less likely formed. Thatis to say, in this mesa with the stepped cross-sectional shape (or withthe gradually and upwardly decreasing planar areas), each semiconductorlayer forms a step. Accordingly, compared to a mesa with no stepped sidefaces (i.e., a mesa that forms only one step by itself), each step ofthis mesa 140 is much lower. Thus, the interconnect 111 can be formed ina desired shape and the pattern failure can be eliminated. In thefabrication process of the first embodiment, the first through thirdetch masks are made of SiN. Alternatively, according to the presentinvention, not just the SiN masks but etch masks made of SiO₂ orphotoresist may also be used.

Embodiment 2

Hereinafter, a second embodiment of the present invention will bedescribed with reference to FIGS. 4 through 6D. FIG. 4 schematicallyillustrates a cross-sectional structure for a photodetector 200according to the second embodiment.

As shown in FIG. 4, the photodetector 200 of the second embodiment isdifferent from the photodetector 100 of the first embodiment in that thephotodetector 200 further includes a carrier barrier layer 120 betweenthe first semiconductor conductive layer 102 and light-absorbing layer103. In the other respects, the photodetectors 200 and 100 have the samestructure, and the description thereof will be omitted or simplifiedherein.

The photodetector 200 shown in FIG. 4 includes photodiode mesa 150 andpad mesa 160 on a semi-insulating InP substrate 101. More specifically,the photodiode mesa 150 includes semiconductor conductive layer 102 ofn-InGaAsP, carrier barrier layer (or buffer layer) 120 of n⁻-InP,light-absorbing layer 103 of n⁻-InGaAs and wide bandgap layer (or InPwindow layer) 104 including a p⁺-type doped region 105. All of theselayers 102, 120, 103 and 104 are stacked in this order on the substrate101 to have upwardly decreasing planar areas. As in the firstembodiment, a level difference exists between the first conductive andbarrier layers 102 and 120 and between the light-absorbing and widebandgap layers 103 and 104. A level difference may also exist betweenthe barrier and light-absorbing layers 120 and 103.

The barrier layer 120, interposed between the first conductive andlight-absorbing layers 102 and 103, prevents carriers, created in theconductive layer 102, from diffusing and entering the light-absorbinglayer 103. Accordingly, the barrier layer 120 can prevent holes,included in electron-hole pairs created in the conductive layer 102through photoexcitation action, from diffusing and entering thelight-absorbing layer 103. The photodetector 200 including this barrierlayer 120 can sense incoming light not only through the principalsurface thereof but also through the backside thereof. This point willbe further detailed below.

FIG. 5 schematically illustrates the bandgaps of the substrate 101 andrespective layers 102, 120, 103 and 104 included in the photodetector200. In FIG. 5, the solid circles indicate electrons while the opencircles indicate holes.

Where the photodetector 200 senses incoming light through the backsideof the InP substrate 101, the incoming light reaches the conductivelayer 102 before entering the light-absorbing layer 103 unlike aphotodetector sensing the incoming light through the principal surfacethereof. The InP substrate 101 has a relatively wide bandgap, andtransmits light with a wavelength of 1.3 or 1.55 μm for use in fiberoptics communications (see FIG. 3 and Table 1). In the conductive layer102 with a bandgap narrower than that of the substrate 101,photoexcitation might occur responsive to the incident light to produceelectron-hole pairs. To prevent holes, included in these electron-holepairs, from diffusing and reaching the light-absorbing layer 103, thebarrier layer 120 of InP, which has a bandgap wider than that of theconductive layer 102, is inserted between the conductive andlight-absorbing layers 102 and 103. In this structure, the holes,included in the electron-hole pairs created in the conductive layer 102,cannot reach the light-absorbing layer 103, but remain in the conductivelayer 102 and are recombined with the electrons there. Accordingly, nophotocurrent will flow.

As shown in FIG. 3 and Table 1, the conductive layer 102 of InGaAsP canhave its absorption edge changed by modifying its composition.Accordingly, where the incoming light enters the photodetector 200through the backside thereof, the conductive layer 102 can function as afilter layer. That is to say, the conductive layer 102 can be providedwith not just electrical conductivity but also the filtering function oftransmitting part of the incoming light with a particular wavelength.

For example, to selectively sense light with a wavelength of 1.55 μmwhile allowing light with a wavelength of 1.3 μm to be absorbed into thelight-absorbing layer 103, InGaAsP as a material for the conductivelayer 102 preferably has an absorption edge longer than 1.3 μm andshorter than 1.55 μm, more preferably longer than 1.35 μm and shorterthan 1.5 μm. As described above, the holes, included in theelectron-hole pairs created in the conductive layer 102 responsive tothe incident light with a wavelength of 1.3 μm, cannot reach thelight-absorbing layer 103 due to the existence of the barrier layer 120,and are recombined with the electrons in the conductive layer 102. As aresult, no photocurrent will be created in response to the light withthe wavelength of 1.3 μm on the other hand, the light with thewavelength of 1.55 μm is transmitted through the conductive layer 102and then absorbed into the light-absorbing layer 103 so as to create aphotocurrent. Accordingly, the photodetector 200 can exhibit aphotosensitivity only to the light with the wavelength of 1.55 μm. Inthis manner, the photodetector 200 can be given wavelength selectivity.

If the light with the wavelength of 1.3 μm, as well as the light withthe wavelength of 1.55 μm, should be sensed, then InGaAsP as a materialfor the conductive layer 102 should have an absorption edge shorter than1.3 μm. Specifically, the absorption edge is preferably longer than 0.93μm and shorter than 1.3 μm, more preferably longer than 0.93 μm andshorter than 1.25 μm. In that case, the light with the wavelength of 1.3μm and the light with the wavelength of 1.55 μm are transmitted throughthe conductive layer 102 and then absorbed into the light-absorbinglayer 103.

In the second embodiment, the conductive layer 102 has a filteringfunction. Accordingly, there is no need to separately provide a filterlayer to give wavelength selectivity to the photodetector. For thatreason, a photodetector with wavelength selectivity is implementableusing a simplified structure, thus cutting down the fabrication cost. Inaddition, the InP substrate 101 and conductive layer 102 have mutuallydifferent etch susceptibilities, so the photodetector 200 ismass-producible as well because of the same reasons described for thefirst embodiment.

In the foregoing embodiment, the barrier layer 120 is made of InP.Alternatively, the barrier layer 120 may be made of any other compoundso long as the layer 120 can prevent the carriers, created in theconductive layer 102, from reaching the light-absorbing layer 103. Also,the pad mesa 160 does not have to be the second conductive layer 109,but may be any other type of layer (e.g., insulating layer). However,the pad mesa 106 is preferably formed out of the second conductive layer109, which is made of the same material as the first conductive layer102, because it is easier to fabricate the photodetector 200.

The photodetector 200 of the second embodiment may be formed in thefollowing specific shape. However, the present invention is not limitedto these specifics but any other appropriate combination may be usedinstead.

The InP substrate 101 may have a thickness of about 150 μm. Thephotodiode mesa 140 may have a height of about 5 μm to about 10 μm. Thefirst conductive layer 102 may have a thickness of about 1.5 μm to about3 μm and may be formed as a rectangular mesa with an area ranging from(40×80) μm² through (100×200) μm². Like the first conductive layer 102,the second conductive layer 109 may also have a thickness of about 1.5μm to about 3 μm. The mesa of the second conductive layer 109 may beeither in a circular shape with a diameter of about 50 μm to about 100μm or in a rectangular (or square) shape, each side of which isapproximately 50 to 100 μm long.

The barrier layer 120 may have a thickness of about 1 μm to about 2 μm,and the mesa thereof may be formed in a similar shape to that of thelight-absorbing layer 103. The light-absorbing layer 103 may have athickness of about 1.5 μm to about 3 μm, and the mesa thereof may be ina circular shape with a diameter of about 35 μm to about 70 μm. The widebandgap layer 104 may have a thickness of about 1 μm to about 2 μm, andthe mesa thereof may be in a circular shape with a diameter of about 30μm to about 60 μm. The doped region 105 in the wide bandgap layer 104may also be in a circular shape with a diameter of about 25 μm to about50 μm. It should be noted that these members may be formed in theexemplified shapes when viewed from over the photodetector 200 (i.e., inthe direction parallel to a normal to the substrate surface).

The carrier densities of these layers 102, 120, 103 and 104 may be asfollows. Specifically, the first conductive layer 102 of n⁺-type mayhave a relatively high carrier density, while the light-absorbing layer103 of n⁻- or i-type may have a relatively low carrier density. In theillustrated embodiment, the wide bandgap and carrier barrier layers 104and 120 are n⁻-type layers with a relatively low carrier density.However, the carrier densities of these layers 104 and 120 are notparticularly limited.

Hereinafter, it will be described with reference to FIGS. 6A through 6Dhow to fabricate the photodetector 200 of the second embodiment. FIGS.6A through 6D are cross-sectional views illustrating respective processsteps for fabricating the photodetector 200.

First, as shown in FIG. 6A, semiconductor conductive layer 102 ofn-InGaAsP, carrier barrier layer 120 of n⁻-InP, light-absorbing layer103 of n⁻-InGaAs, and wide bandgap layer 104 of n⁻-InP are stacked inthis order on a semi-insulating semiconductor substrate 101 of InP by acrystal growth process. Next, part of the wide bandgap layer 104 isdoped with a dopant (e.g., Zn) that reaches the light-absorbing layer103, thereby defining a doped region 105. As described above, infabricating a photodetector 200 that senses the light with a wavelengthof 1.55 μm selectively through the backside thereof, InGaAsP as amaterial for the conductive layer 102 preferably has an absorption edgelonger than 1.3 μm and shorter than 1.55 μm, more preferably longer than1.35 μm and shorter than 1.5 μm.

Next, as shown in FIG. 6B, a first etch mask 301, covering the dopedregion 105, is defined on the wide bandgap layer 104 and then the widebandgap layer 104 is selectively etched while being masked with thefirst mask 301. The first etch mask 301 may be made of SiN, for example.In the illustrated embodiment, when the wide bandgap layer 104 should beetched, a first etchant, realizing an etch selectivity of 10 or more forthe wide bandgap layer 104 of InP against the light-absorbing layer 103of InGaAs, is used. Examples of such etchants include a mixture ofhydrochloric acid and phosphoric acid with a volume ratio of 1 to 4.Thereafter, the first mask 301 is removed.

Then, as shown in FIG. 6C, a second etch mask 302, covering the widebandgap layer 104 including the doped region 105, is defined on aselected area of the light-absorbing layer 103. Subsequently, thelight-absorbing and barrier layers 103 and 120 are etched selectivelywhile being masked with the second mask 302, which may also be made ofSiN.

In the illustrated embodiment, when the light-absorbing layer 103 ofInGaAs should be etched, a second etchant, realizing an etch selectivityof 10 or more for the light-absorbing layer 103 of InGaAs against thebarrier layer 120 of InP, is used. Examples of such etchants include amixture of sulfuric acid, hydrogen peroxide water and water with avolume ratio of 1 to 1 to 5. On the other hand, when the barrier layer120 of InP should be etched, a third etchant, realizing an etchselectivity of 10 or more for the barrier layer 120 of InP against theconductive layer 102 of InGaAsP, is used. Examples of such etchantsinclude a mixture of hydrochloric acid and phosphoric acid with a volumeratio of 1 to 4. Thereafter, the second mask 302 is removed.

Subsequently, as shown in FIG. 6D, a third etch mask 303 is defined onthe conductive layer 102 to cover not only the wide bandgap,light-absorbing and carrier barrier layers 104, 103 and 120 but alsopart of the conductive layer 102 where the mesa-etched light-absorbinglayer 103 does not exist. And then the conductive layer 102 is maskedwith this third mask 303 and partially etched away. The third mask 303may also be made of SiN.

In the illustrated embodiment, when the conductive layer 102 of InGaAsPshould be etched, a fourth etchant, realizing an etch selectivity of 10or more for the conductive layer 102 of InGaAsP against the substrate101 of InP, is used. Examples of such etchants include a mixture ofsulfuric acid, hydrogen peroxide water and water with a volume ratio of1 to 1 to 5.

By performing these etch process steps, a photodiode mesa 150 withstepped side faces and a pad mesa 160 made of the second conductivelayer 109 are formed. Thereafter, the third mask 303 is removed. Andthen an insulating film 108 of SiN, for example, is deposited over thesubstrate except the area where the doped region 105 should be exposedand the area where an n-side electrode 107 will be formed.

Thereafter, a negative photoresist pattern for p- and n-side electrodes106 and 107, pad 110 and interconnect 111 is defined. Next, a stack ofTi, Pt and Au films is deposited over the substrate and then thephotoresist pattern is lifted off, thereby forming the p- and n-sideelectrodes 106 and 107, pad 110 and interconnect 111. In this manner,the photodetector 200 shown in FIG. 4 is completed.

In the fabrication process of the second embodiment, the conductivelayer 102 of InGaAsP with etch susceptibility different from that of theInP substrate 101 is formed on the substrate 101, and can be etchedselectively with respect to the substrate 101. Thus, the etch process iscontrollable much more easily. As a result, the photodiode mesa 150 canhave its height minimized.

In addition, the respective semiconductor layers are etched selectivelyusing the three etch masks in mutually different shapes. By using thesemultiple etch masks, the respective semiconductor layers 102, 120, 103and 104 can be stacked one upon the other so that the photodiode mesa150 has stepped side faces. As a result, the interconnect 111 can beformed in a desired shape and the disconnection thereof is avoidable.Furthermore, in the fabrication process of the second embodiment, thelight-absorbing and barrier layers 103 and 120 are both masked with thesecond mask 302 and etched using the second and third etchants,respectively. Accordingly, it takes a reduced number of process steps todefine the etch masks.

Embodiment 3

Hereinafter, a third embodiment of the present invention will bedescribed with reference to FIGS. 7 through 8D. FIG. 7 schematicallyillustrates a cross-sectional structure for a photodetector 300according to the third embodiment.

As shown in FIG. 7, the photodetector 300 of the third embodiment isdifferent from the photodetector 100 of the first embodiment in that theinsulating film 114 as an undercoat for the interconnect 111 is a stackof SiN and SiO₂ layers in the photodetector 300. In the other respects,the photodetector 300 has the same structure as the photodetector 100,and the description thereof will be omitted or simplified herein.

The photodetector 300 shown in FIG. 7 includes photodiode mesa 140 andpad mesa 160 on a semi-insulating InP substrate 101. More specifically,the photodiode mesa 140 includes semiconductor conductive layer 102 ofn-InGaAsP, light-absorbing layer 103 of n³¹ -InGaAs and wide bandgaplayer (or InP window layer) 104 including a p⁺-type doped region 105.All of these layers are stacked in this order on the substrate 101 tohave upwardly decreasing planar areas. As in the first embodiment, alevel difference exists between the conductive and light-absorbinglayers 102 and 103 and between the light-absorbing and wide bandgaplayers 103 and 104. It should be noted that a carrier barrier layer 120may be additionally formed between the conductive and light-absorbinglayers 102 and 103 as in the second embodiment. Also, the pad mesa 160does not have to be the second conductive layer 109, but may be anyother type of layer (e.g., insulating layer).

In the photodetector 300, an insulating film 114, consisting ofpassivation film 108 of SiN and interlevel dielectric film 112 of SiO₂,is deposited over the substrate except the area where the doped region105 should be exposed and the area where an n-side electrode 107 isformed. More specifically, the SiN passivation film 108 (which will besimply referred to as an “SiN layer”) has been-deposited to cover thesurfaces of the photodiode and pad mesas 140 and 160 and the exposedsurface of the substrate 101. And the SiO₂ interlevel dielectric film112 (which will be simply referred to as an “SiO₂ layer”) has beendeposited on the SiN layer 108. In the illustrated embodiment, the SiNlayer 108 may have a thickness of about 30 nm and the SiO₂ layer 112 mayhave a thickness of about 500 nm.

In the photodetector 300 of the third embodiment, the insulating film114, covering the surface of the photodiode mesa 140, has a multilayerstructure and is thicker than the single SiN layer 108 for the first orsecond embodiment. Accordingly, the interconnect capacitance can bereduced. Hereinafter, it will be described why the insulating film 114is formed as a stack of the SiN and SiO₂ layers 108 and 112.

Generally speaking, the thickness of an SiN layer should not exceed acertain limit because cracks would be formed easily in a thick SiNlayer. In addition, an SiN layer has a dielectric constant higher thanthat of an SiO₂ layer. However, it is also known that where apassivation film for an InP photodetector is made of SiO₂, a leakagecurrent, flowing through the photodetector, increases compared to astructure in which the photodetector includes a passivation film of SiN.Thus, the present inventor believed that if the insulating film isformed as a stack of a thick SiO₂ layer for reducing the capacitance ona thin SiN layer for passivation purposes, then a photodetectorincluding that insulating film should have its leakage current andinterconnect capacitance both reduced. And as a result of experiments Icarried out, I succeeded in making a photodetector 300 with aninterconnect capacitance that had been reduced to about half of thejunction capacitance thereof. Specifically, the photodetector 300 shownin FIG. 7 had a junction capacitance of 0.1 pF and an interconnectcapacitance of 0.05 pF.

The SiN layer 108 preferably has a thickness of 20 nm through 100 nm.This is because where the SiN layer 108 has a thickness of 20 nm ormore, good passivation effects are attainable. Also, if the thickness ofthe SiN layer 108 is 100 nm or less, no cracks will be made in the SiNlayer 108. On the other hand, where a photodetector, including a dopedregion 105 with a diameter of about 35 μm, should have its interconnectcapacitance halved compared to the junction capacitance thereof, theSiO₂ layer 112 preferably has a thickness of 400 nm or more. The SiO₂layer 112 has no maximum allowable thickness, but may be of anythickness appropriate for the photodetector 300.

Hereinafter, it will be described with reference to FIGS. 8A through 8Dhow to fabricate the photodetector 300 of the third embodiment. FIGS. 8Athrough SD are cross-sectional views illustrating respective processsteps for fabricating the photodetector 300 of the third embodiment.

First, the same process steps as those illustrated in FIGS. 2A through2D are carried out, thereby obtaining the structure shown in FIG. 8A, inwhich the photodiode and pad mesas 140 and 160 are formed on thesemi-insulating InP substrate 101. The structure shown in FIG. 8A isformed by the fabrication process of the first embodiment. Thus, theeffects of the first embodiment are also attainable by the thirdembodiment.

Next, as shown in FIG. 8B, a passivation film 108 of SiN and aninterlevel dielectric film 112 of SiO₂ are deposited in this order overthe substrate to thicknesses of about 30 nm and about 500 nm,respectively, so as to cover the photodiode and pad mesas 140 and 160.Then, parts of the interlevel dielectric and passivation films 112 and108, in which the doped region 105 should be exposed and an n-sideelectrode 107 will be formed, respectively, are etched away, therebyforming openings 310. It should be noted that the parts of theinsulating film 114 located over the photodiode mesa 140 cover theformerly exposed surfaces of the wide bandgap, light-absorbing andconductive layers 104, 103 and 102 and also have stepped side facescorresponding to those of the photodiode mesa 140.

Subsequently, as shown in FIG. 8C, a spacer film 311 of SiN is depositedto a thickness of about 200 nm over the interlevel dielectric film 112and then a negative photoresist pattern 312 for p- and n-side electrodes106 and 107, pad 110 and interconnect 111 is defined thereon. And thespacer film 311 is partially etched away using the photoresist pattern312 as a mask. The spacer film 311 is inserted between the undercoatinsulating film 114 and the negative photoresist pattern 312. Byinserting this spacer film 311, it is possible to prevent the patternedmetal thin film from being lifted off unintentionally along with themetal thin film deposited on the photoresist pattern 303.

Then, as shown in FIG. 8D, Ti, Pt and Au films are deposited in thisorder over the substrate to respective thicknesses of about 50 nm, about100 nm and about 400 nm and the photoresist pattern 312 is dissolved inacetone, for example, and lifted off. In this manner, the p- and n-sideelectrodes 106 and 107, pad 110 and interconnect 111 are formed at atime.

Thereafter, the spacer film 311 is removed, thereby completing thephotodetector 300 shown in FIG. 7. Optionally, the spacer film 311 maybe left as it is and used as an antireflection film for a photodetectorof the type sensing light incoming through the principal surfacethereof. As another alternative, an antireflection film may be newlydeposited after the structure shown in FIG. 7 has been once obtained.

In the fabrication process of the third embodiment, the spacer film 311is used. Accordingly, compared to using a photoresist pattern alone, thestack of the Ti, Pt and Au films can be lifted off more easily. Also,the spacer film 311 is made of SiN and the underlying interleveldielectric film 112 is made of SiO₂, so the spacer film 311 can beetched selectively. More specifically, where a reactive ion etch processis carried out using a reactive gas like CF₄, the etch rate of SiN ismuch higher than that of SiO₂. Accordingly, if the spacer film 311 isremoved by a reactive ion etch process, only the spacer film 311 isremovable almost without etching the interlevel dielectric film 112.

In the photodetector 300 of the third embodiment, the pad electrode 110is formed on the pad mesa 160, which is located in a different area fromthat of the photodiode mesa 140. Accordingly, no parasitic capacitanceis associated with the pad electrode 110. In addition, the parasiticcapacitance formed between the photodiode mesa 140 and interconnect 111can also be reduced. As a result, the photodetector 300 can operate muchfaster than the known photodetector. Optionally, the third embodimentmay be combined with the second embodiment. In that case, the effects ofthe second embodiment are also attainable.

What is claimed is:
 1. A photodetector comprising: a semi-insulatingsemiconductor substrate; a semiconductor conductive layer, which hasbeen formed on a surface region of the substrate and has electricalconductivity; a light-absorbing layer, which has been formed on theconductive layer and absorbs light that has been incident on thephotodetector; a wide bandgap layer, which has been formed on thelight-absorbing layer and has a bandgap wider than a bandgap of thelight-absorbing layer; and a doped region, which has been defined in thewide bandgap layer by doping part of the wide bandgap layer with adopant that reaches the light-absorbing layer, wherein the conductivelayer has etch susceptibility different from that of the substrate. 2.The photodetector of claim 1, wherein the substrate is made of InP, theconductive layer is made of InGaAsP, the light-absorbing layer is madeof InGaAs, and the wide bandgap layer is made of InP.
 3. Thephotodetector of claim 1, wherein InGaAsP as a material for theconductive layer has an absorption edge longer than 0.93 μm and shorterthan 1.55 μm.
 4. The photodetector of claim 1, wherein the conductivelayer is an n-type semiconductor layer, and wherein the dopant is ap-type dopant, and wherein the light-absorbing layer functions as anintrinsic layer of a pin photodiode, and wherein the photodetectorfurther comprises: an n-side electrode, which makes an electricalcontact with the conductive layer; and a p-side electrode, which makesan electrical contact with the doped region.
 5. The photodetector ofclaim 1, wherein a semiconductor multilayer structure, including thesemiconductor conductive, light-absorbing and wide bandgap layers, hasbeen formed on said surface region-of the substrate, and wherein asecond semiconductor conductive layer has been formed on another surfaceregion of the substrate and is electrically isolated from the conductivelayer included in the multilayer structure, and wherein a pad for use toelectrically connect the photodetector to an external unit has beenformed on the second conductive layer, and wherein the pad iselectrically connected to the doped region that has been defined in saidpart of the wide bandgap layer in the multilayer structure.
 6. Thephotodetector of claim 5, wherein a ring electrode with an opening atthe center thereof has been formed on the doped region, and wherein thering electrode is connected to the pad by way of an interconnect thathas been formed on an insulating film, the insulating film covering thesurface of the multilayer structure.
 7. The photodetector of claim 5,wherein the semiconductor conductive, light-absorbing and wide bandgaplayers, making up the multilayer structure, have been stacked one uponthe other to make a level difference exist between each of these layersand an adjacent one of the layers.
 8. A photodetector comprising: asemi-insulating semiconductor substrate; a semiconductor conductivelayer, which has been formed on a surface region of the substrate andhas electrical conductivity; a light-absorbing layer, which absorbslight that has been incident on the photodetector; a carrier barrierlayer, which has been formed between the conductive and light-absorbinglayers to prevent carriers, created in the conductive layer, fromdiffusing and entering the light-absorbing layer; a wide bandgap layer,which has been formed on the light-absorbing layer and has a bandgapwider than a bandgap of the light-absorbing layer; and a doped region,which has been defined in the wide bandgap layer by doping part of thewide bandgap layer with a dopant that reaches the light-absorbing layer,wherein the conductive layer is made of InGaAsP and transmits part ofthe incident light with a particular wavelength.
 9. The photodetector ofclaim 8, wherein InGaAsP as a material for the conductive layer has anabsorption edge longer than 1.3 μm and shorter than 1.55 μm.
 10. Thephotodetector of claim 9, wherein the absorption edge is longer than1.35 μm and shorter than 1.5 μm.
 11. The photodetector of claim 8,wherein InGaAsP as a material for the conductive layer has an absorptionedge longer than 0.93 μm and shorter than 1.3 μm.
 12. The photodetectorof claim 11, wherein the absorption edge is longer than 0.93 μm andshorter than 1.25 μm.
 13. The photodetector of claim 8, wherein thesubstrate is made of InP, the conductive layer is made of InGaAsP, thebarrier layer is made of InP, the light-absorbing layer is made ofInGaAs, and the wide bandgap layer is made of InP.
 14. The photodetectorof claim 8, which senses light that has been incident on thephotodetector through a backside of the substrate.
 15. The photodetectorof claim 8, wherein a semiconductor multilayer structure, including thesemiconductor conductive, carrier barrier, light-absorbing and widebandgap layers, has been formed on said surface region of the substrate,and wherein a second semiconductor conductive layer has been formed onanother surface region of the substrate and is electrically isolatedfrom the conductive layer included in the multilayer structure, andwherein a pad for use to electrically connect the photodetector to anexternal unit has been formed on the second conductive layer, andwherein the pad is electrically connected to the doped region that hasbeen defined in said part of the wide bandgap layer in the multilayerstructure.
 16. A photodetector comprising: a semi-insulatingsemiconductor substrate; a semiconductor conductive layer, which hasbeen formed on a surface region of the substrate and has electricalconductivity; a light-absorbing layer, which has been formed on theconductive layer and absorbs light that has been incident on thephotodetector; a wide bandgap layer, which has been formed on thelight-absorbing layer and has a bandgap wider than a bandgap of thelight-absorbing layer; a doped region, which has been defined in thewide bandgap layer by doping part of the wide bandgap layer with adopant that reaches the light-absorbing layer; and an electrode, whichhas been formed on the doped region, wherein a semiconductor multilayerstructure, including the semiconductor conductive, light-absorbing andwide bandgap layers, has been formed on said surface region of thesubstrate, and wherein a second semiconductor conductive layer has beenformed on another surface region of the substrate and is electricallyisolated from the conductive layer included in the multilayer structure,and wherein a pad for use to electrically connect the photodetector toan external unit has been formed on the second conductive layer, andwherein the multilayer structure is covered with an insulating film, andwherein an interconnect has been formed on the insulating film toelectrical connect the electrode and the pad together, and wherein theinsulating film is a stack of an SiN layer and an SiO₂ layer that hasbeen deposited on the SiN layer.
 17. The photodetector of claim 16,wherein the SiN layer has a thickness of 20 nm through 100 nm, andwherein the SiO₂ layer has a thickness of 400 nm or more.
 18. Thephotodetector of claim 16, further comprising a carrier barrier layerbetween the conductive and light-absorbing layers, the barrier layerpreventing carriers, created in the conductive layer, from diffusing andentering the light-absorbing layer.